The Swedish Patent No. 355 719 discloses a capacitive sensor capable of sensing the immediate proximity of a person's finger to an electrical conductor extended along a wall. It represents a first step towards developing a multi purpose device which is suitable for indicating close proximity of a person to a surface along an object such as a truck or a painting. In the known sensor, a voltage measuring device is provided with a signal ground connected to a first pole of an AC power line that has a second pole connected to earth and that is connected to an input of the voltage measuring device via a voltage divider with a capacitive series link arranged between said input and earth and a shunt link arranged between said input and said signal ground. While it is obvious that the AC power line could be replaced by a battery powered oscillating voltage source and that the man skilled in the art also could improve the voltage measuring device, the connection to earth of the voltage source creates a problem. The known sensor is not sensitive unless the impedance of this connection is considerably lower than the capacitive impedance to earth presented by the object at which close proximity is to be indicated. This is easy to arrange for the entire surface of trucks only when they are parked, and a wired connection to earth limits the use of capacitive sensors for protecting paintings in museums or for providing all kinds of objects with a proximity detection obtained at a suitable reaction distance.